求详解为什么选A,不选B!If your typical text costs $100 for a new edition,used ______ $75,an e-book $55,and a rental $45.A will cost Bcost

求详解为什么选A,不选B!
If your typical text costs $100 for a new edition,used ______ $75,an e-book $55,and a rental $45.
A will cost Bcost

假设的虚拟语气,当然是选A；前面有If,Unless等,都是；这里used是被使用者意思,不是动词